1. Field of the Invention
The present invention relates to a variable valve timing controller for an engine.
2. Description of the Related Art
A wide variety of techniques have been developed and proposed in order to reduce the amount of hydrocarbon (HC) contained in the exhaust gas from an internal combustion engine. For example, Japanese Laid-Open Patent Publication No. 2002-13419 discloses a method in which exhaust gas are discharged through an intake port and an exhaust port by setting a valve overlap period (during which both an intake-valve and an exhaust-valve are open) before top dead center (TDC) (that is, before a piston reaches TDC on an exhaust stroke). It is also disclosed that with this method, the amount of HC discharged can be reduced.
Typically, HC discharged from the engine can be removed by chemically reacting the HC in a catalyst. In this case, a catalyst must be raised to a predetermined high temperature so that it is activated. If an ignition timing is retarded to raise the temperature of exhaust gas, the temperature of a catalyst can be made high.
However, the retardation of the ignition timing causes combustion within an engine cylinder to be unstable. If the amount of an internal exhaust gas recirculation (EGR) gas (i.e., an exhaust gas remaining within the engine cylinder) is reduced, combustion stability can be enhanced. The internal EGR gas within the engine cylinder can be minimized by setting a valve-overlap period to zero (i.e., when the piston reaches TDC, an exhaust-valve closes and an intake-valve opens).
Thus, with a rise in the temperature of exhaust gas by the retardation of the ignition timing and an early activation of the catalyst by the temperature increase of exhaust gas, HC generated immediately after the engine has started (particularly, when the engine is under cold conditions) can be processed by the catalyst. At the same time, engine combustion stability can also be assured.
However, if the amount of the internal EGR gas is minimized as described above, the amount of HC discharged from the engine will be increased. This problem will hereinafter be described with reference to FIG. 8.
FIG. 8a shows opening and closing timing of the intake-valve and the exhaust-valve as a valve-lift characteristic. The “EO” shown in the figure stands for the exhaust-valve opening; the “EC” stand for the exhaust-valve closing; the “IO” stands for the intake-valve opening; and the “IC” stands for the intake-valve closing. According to the valve lift curves shown in FIG. 8a, the internal EGR gas within the engine cylinder can be minimized by closing the exhaust-valve and opening the intake-valve, when the piston reaches TDC. That is, the internal EGR gas within the engine cylinder can be minimized by setting the valve-overlap period to zero. In this setting, the exhaust-valve and the intake-valve do not simultaneously open.
However, if the exhaust-valve and the intake-valve are operated along with the curve as shown in FIG. 8a, the amount of HC contained in the exhaust gas from the engine increases rapidly when the exhaust-valve opens (see arrow A1), as shown in FIG. 8b. Thereafter, it is gradually reduced, but increases again rapidly when before TDC (before the exhaust-valve is closed) (see arrow A2). As a result, HC discharged from the engine to the catalyst is increased if the temperature of a catalyst is raised in order to enhance processing ability of the catalyst and assure engine combustion stability.